In general, an ink reservoir for a writing instrument molded from a gas barriering resin is used for writing instruments charged with a fragrant ink and a volatile ink for the purpose of maintaining fragrance and controlling volatilization of the inks.
A gas barriering resin used for this ink reservoir for a writing instrument has a hydrophilic group in a molecule thereof in many cases, and it is a hydrophilic resin, so that moisture absorption takes place due to moisture contained in the air and volatilization of moisture contained in an ink. As a result thereof, the problem that a gas barriering property and a mechanical strength are reduced is caused. In particular, it is known that the gas barriering property is exponentially reduced as moisture is absorbed.
Accordingly, when a hydrophilic gas barriering resin is used for members for a writing instrument, the outside thereof has to be covered with a hydrophobic resin such as polypropylene (PP) to cut off moisture contained in the air.
However, further addition of a molded holder member to the outsides of a holder and an ink reservoir of a writing instrument leads to an increase in the holder diameter and brings about the problem that the use feeling and the carrying property are reduced.
On the other hand, refills having various structures have so far been proposed for a ballpoint pen refill comprising an ink reservoir. Provided is, for example, a ballpoint pen refill of a so-called pressure type which is equipped with an ink reservoir and a ballpoint pen tip fixed at one side of this ink reservoir and in which a side opposite to the tip is tightly closed with a plug, wherein the tip side of the ink reservoir is charged with an ink, and the side opposite to the tip in the ink reservoir is charged with pressurized gas; and the ink is pushed out toward a direction of the tip by this pressurized gas.
In such ballpoint pen refill of a pressure type, a force to push out an ink toward a tip direction is always exerted on the ink due to a positive pressure state, and therefore writing is possible in the state that the tip is turned upward. Accordingly, back flow of an ink caused in writing in an upward state with a ballpoint pen refill of a non-pressure type in which a side opposite to the tip is opened is not caused in the ballpoint pen refill of a pressure type, and it is suited to writing with the tip turned upward in writing on paper adhered on a wall or writing while holding a clip board with a hand.
In order to maintain a pressure force over a long period of time in the ballpoint pen refill of a pressure type described above, a metal-made ink reservoir having a gas non-permeable wall is used or in a resin-made ink reservoir, measures are assumed so that the reservoir is increased in a thickness to prevent a reduction in a pressure force due to permeation of gas. However, in the metal-made ink reservoir, the inside of the ink reservoir can not be observed, and therefore a remaining amount of the ink can not be confirmed. Further, it has the defect that it is expensive. On the other hand, the resin-made ink reservoir has the advantage that an ink reservoir which is inexpensive and has a desired form is liable to be obtained. However, in order to raise the gas barriering property so that it can be used for a ballpoint pen refill of a pressure type, the reservoir has to be increased in a thickness, and therefore an ink remaining amount is less liable to be confirmed depending on the material. Further, there are the problems that freedom in designing the members is low and that shrinking and bending are liable to be caused.
Further, use of resins comprising an ethylene-vinyl alcohol copolymer, for example, an ethylene-vinyl alcohol copolymer, alloys or blends of the above copolymer with polyolefins or gas scarcely barriering resin compositions comprising a composite as a material for the resin-made ink reservoir makes it possible to decrease a thickness of the resin-made ink reservoir described above to some extent. A resin wall formed from a composition comprising these ethylene-vinyl alcohol copolymers can inhibit well gas permeation under dry environment, but under environment such as humidified atmosphere, the resin is swollen by moisture contained in the air, and the gas barriering property is reduced. In addition thereto, a dimensional change is caused, so that brought about are the problems that the tip on which a pressure force is always exerted is liable to fall out and that the ink is liable to leak.